Peak clipper



"\iVENTOR. I WILLIAM W. H. DEAN A ji hawk.

AT'II'ORNEY w. w. H. DEAN PEAK CLIPPER Fil ed'Aug. 1a, 1947 AAA June 5, 1951 Patented June 5, 1951 PEAK CLIPPER William W. H. Dean, Pointe Claire, Quebec, Ganada, assignor to Radio Corporation of America, a corporation of Delaware Application August 13, 1947, Serial No. 768,319

Claims.

1 The purpose of the invention, I am about to describe is first to increase the average modulation level of a speech modulated carrier without increasing the peak amplitude of the modulation and to provide in a simple and accurate manner, means for adjusting the optimum clipping point. The point may be selected as desired for the particular communication system involved.

To attain the first purpose, I devised a simple inexpensive peak clipper which is of low initial cost and inexpensive in operation and upkeep. My clipper uses but two tube structures which may be in a single envelope. In operation, the D.-C. plate current drain is sufliciently low that it can usually be obtained from existing equipment thereby eliminating the need for a separate high voltage supply. Although as stated above, my clipper is simple and of low cost, it is highly eflicient, linear to the point of clipping and clips cleanly and symmetrically on both positive and negative peaks.

My clipper forms a unit which may with advantage replace clippers in existing equipment using the same or may be used in equipment not now using a clipper. Then improvement in communication equal to a power increase of 5 or times is obtained with substantially no actual increase in power used.

Other advantages to be attained by the use of j my invention appear from a detailed description which follows. In the description which follows, reference will be made to the attached drawing wherein the single figure illustrates a peak clipping amplifier arranged in accordance with my invention.

In the drawing, the rectangle l0 designates a pre-amplifier in which the modulation is amplified as desired. This amplifier may be conventional and includes in its output a level control potentiometer l2, the tap on which is connected to the control grid 2| of an electron discharge tube having an anode 23 and a cathode 22. This electron discharge tube may be in a separate envelope or in an envelope with a second electron discharge device having its anode 29 connected to a direct current source by a plate load resistor R. The cathode return resistor 24 is connected between the cathodes of both tubes and ground. The control grid 28 of the second tube is driven by voltages produced in the cathode resistor 24 because such control grid is connected to the grounded end of resistor 24 through a resistor 32.

The tube structures, as described, constitute a 2 peak limiter of the type disclosed in Crosby Patent 2,276,565 dated March 17, 1942. In accordance with my invention, however, the anode of the second tube is coupled by condenser C and resistors 30 and 32 to the common grounded point and a pointbetween the resistors 30 and 32 is connected to the control grid 28 to provide degeneration in the system to improve linearity in the amplification up to the clipping point. Clipping introduces harmonic distortion and the clipped voltages are fed through a low pass filter LPF before further amplification in the modulator and driver MD. The filter is necessary to remove higher order harmonics of the speech voltages generated by the clipper action.

In operation, the peak clipper is in physical makeup, similar in part at least, to the limiting amplifier described by Crosby in his Patent 2,276,-

665 dated March 1'7, 1942. In the description that follows, it will be assumed that the reader is familiar with the operation of Crosbys limiter. My clipper is to be used on speech signals and the operation obviously must be linear up to the clipping point. To obtain such operation, de-

r generative feedback circuit for the last tube stage including resistors 35 and 32 is provided. The resistors 36, 32 are un-bypassed so that signal voltages are fed to grid 28. This reduces the distortion which would take place in the clipper without such feedback. The degree of clipping is adjusted by movement of the top point on potentiometer l2, as clipping starts when the plate current of the tubes becomes zero. The use of feedback extends the linear portion of the tube characteristic as zero plate current is approached. In operation, then, both grids are at ground potential in the absence of signals, since both tubes then are conducting, as governed by the bias placed on the grids by cathode resistor 24. The current in the tubes is then insufi'icient to bias the tubes to cut-off. When signal is applied, the positive cycles on grid 2| cause the cathode current to increase and the increase in potential drop in resistor 24 makes the cathode more positive relative to ground, which has the same effect as making the grid 28 more negative and grid limiting in the positive halves of the modulation cycles takes place in this tube structure. The negative potential of the modulation cycles drives grid 2! to cut off, so that full wave limiting takes place in the tube. The amplification, however, is linear, for reasons pointed out above, up to the point at which clipping takes place. As grid 28 approaches the negative potential at which cutoff would take place because of the potential drop in resistor 24, the plate current variation is almost linear with respect to the grid voltage variation because of the feedback from anode 29 to grid 28. The first tube is connected as a cathode follower and hence 100% feedback exists at grid 2|; therefore, linearity on negative half cycles is good.

Using the constants listed below the clipper stage has a voltage gain (below clipping) "of 5.

Resistor 12, 1 meg. Resistor R, 47,000 ohms Resistor 30, 270,000 ohms Resistor 32, 2,700 ohms Resistor 24, 2,700 ohms Clipper tube, type 6SL7GT What is claimed is:

1. In a voltage amplifier and voltage peak clipper wherein amplification is substantially linear to the clipping point, a voltage peak limiter comprising two tube structures each havingian anode, a control r'grid and a cathode with a biasing circuit :for the control grids including a common resistor in the cathode return circuit and'a separate impedance connecting each grid to the end of-said common resistor remote from the oathodes, .means for applying voltages to be clipped :to the control grid of one tube structure, means for .deriving clipped voltages from the enode of the other tube structure, means for supplying unidirectional.operating potentials to the anodes of both tube structures, and a feedback path connecting .the :anode of the other tube structure to itscontrol grid.

2. In a voltage'arnplifier and voltage peak clipper wherein amplification is substantially linear to-the-clipping points, voltage peak limiter comprising-two tube structures each having ananode,

-a control grid and acathode with a biasing circuit for the control grids including a common resistor in thecathode return circuit and a separate impedance-connecting each grid to the end 'of saidcommon resistor remote from the oathodes, one of said impedances being adjustable "and being connected to apply voltages to be "clipped to the-control grid of one tube structure, an impedance connected to the anode of the other tube structure for deriving clipped voltages from the anode of said othertube structure, means for supplying unidirectional operatconnecting each control electrode to the end of said common resistor remote from the emitting electrodes, means for applying voltages to be clipped to the control electrode of one electron path, means for deriving clipped voltages from the receiving electrode of the other electron path, means for supplying unidirectional operating potentials to the receiving electrodes of both-elec-. --tr0n paths, and a'feedback path connecting the --receivin'g electrode of said other electron path to thecontrol electrode thereof, said feedback :path including a series impedance.

at. A :voltage amplifier and voltage peak clipper in ac'cordancewith claim 3 wherein the impedance included in the feedback path consists of a condenser and a resistor in series.

5. A-voltage amplifier and voltage peak clipper in accordance with claim 3 wherein said means .forderiving includes a low pass filter coupling the receiving electrode of said other electron :path to output terminals.

WILLIAM W. H. DEAN.

REFERENCES CITED -The following references are of record in the file of this patent:

UNITED STATES PATENTS Number v Name Date "2,050,852 Moore Aug. 11, 1931 2,264,724 'Schonfel'd Dec. 2, 1941 2,276,565 Crosby Mar. 17, 1942 2305;845 Cockerell Dec. 22, 1942 2,403,988 Likel July 16, 1946 2,405,845 Pfaff Aug. 13, 1946 '2;423,263 Sprague July 1, 1947 

